Valediction is the eleventh in Robert B Parker's Spenser series. It has dropped to $2.99 at Kindle and Kobo US.
Kindle US:
https://www.amazon.com/dp/B002SVQD5K
Kindle US/Smile:
https://smile.amazon.com/dp/B002SVQD5K
Kobo US:
https://www.kobo.com/us/en/ebook/valediction